The present invention generally relates to thermal imaging devices and systems, and, more specifically, relates to infrared detector arrays used in such thermal imaging systems.
Thermal imaging systems are used to convert a focused radiation image, principally in the infrared spectral region, of the temperature and thermal emissivity differences within a given scene into a visible picture. In such systems, the image may be scanned region-by-region over one or more detector devices or elements which transform the infrared radiation into an electrical signal. After suitable amplification and electronic processing, this signal can be used to energize an electrooptic transducer or display, such as a cathode ray tube, to provide a visual picture. The detector elements can be made from a semiconductor material, such as mercury cadmium telluride, so that the electrical signal is obtained from a photo-current consisting of free electrons and holes liberated from the bound molecular structure of the material by the infrared photons.
One such system employs a single detector element over which the whole image is scanned; but improved performance is obtained by using a plurality of detector elements, usually in a line (linear array). The image may be scanned and the elements arranged in such a way that each element samples a separate part of the same image, and, therefore, operates on a reduced frequency bandwidth, providing an overall improvement in signal-to-noise ratio as compared with a single element detector. This mode of operation is known as the "parallel scan" mode. Alternatively, the image may be scanned and the elements arranged in such a way that each region or spot of the image is focused onto each element in turn. The signals detected by the individual elements are added together so as to correlate with one another, but the noise associated with each is uncorrelated. Thus, this mode of operation, which is known as the "serial scan" mode, also provides an overall improvement in signal-to-noise ratio.
For both the parallel and serial scan mode type of systems, it is necessary to provide at least one electrical lead for each detector element, plus one common lead from the cooling vessel. The number of electrical leads involved consequently makes encapsulation of the detector elements difficult and expensive to provide. A detector device which minimizes the number of electrical leads required to be brought out therefrom is shown in U.S. Pat. No. 3,995,159, issued Nov. 30, 1976, entitled "Thermal Imaging Systems," the inventor of which is Charles Thomas Elliot. Such patent describes a single three-electrode linear detector which replaces the conventional series or parallel linear detector array in a scanned image thermal detection system. Such detector described in such patent comprises an elongated semiconductor/photoconductor strip of mercury cadmium telluride. A bias current in the strip is arranged to give a photocarrier drift velocity in the strip which matches the image scanning velocity, thereby giving enhanced image resolution. Modulation and photocarrier current which constitutes the detected image is measured as a resistivity change between two readoutelectrodes positioned at one end of the elongated semiconductor/photoconductor strip. One problem associated with such elongated semiconductor/photoconductor strip is the time required for signal readout at the two readout electrodes positioned near one end of the strip.
It is, accordingly, a primary object of the present invention to provide a single detector in the form of an elongated semiconductor/photoconductor strip which includes a mechanism for decreasing the time for signal readout of such strip.